feat: slippage-fee-calculation

[pradeep-selvaraju]

Note

Medium Risk
Changes how V2 BUY orders are sized when user_usdc_balance is provided, which can alter on-chain order amounts and potentially reject orders that previously succeeded. Risk is mitigated by strict validation, defaulting to no padding, and extensive unit/integration tests.

Overview
Adds a new fee_slippage percentage (default 0) at both Config and OrderBuilder levels to pad the platform-fee reserve for BUY orders, with validation that it is 0 or in [1, 100].

Introduces clob::fees with shared fee-reservation logic and updates V2 BUY limit and market order building to conservatively shrink notional/size/amount when user_usdc_balance is set so principal + (padded platform fee) + builder fee ≤ balance, erroring if truncation would produce a zero-sized order. Existing adjust_market_buy_amount is refactored to delegate to the new logic (with slippage disabled), and tests are updated/added to lock in the new behavior.

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